Glutamate as transmitter of hippocampal perforant path

Effects of zinc on markers of glutamate and aspartate neurotransmission in rat hippocampus

Receptor binding and synaptosomal uptake of L-[3H]glutamate and L-[3H]aspartate were measured in hippocampus derived from rats maintained on zinc restricted diets from weaning. Despite near lethal zinc deficiency, these markers of excitatory amino acid neurotransmission were unaffected compared to zinc-supplemented controls. However, addition of zinc in vitro markedly inhibited binding of glutamate and aspartate to hippocampal membranes. These data suggest that zinc can modulate the receptor affinities for glutamate and aspartate and may function as a tonic inhibitor of excitatory synapses in vivo.

2-Amino-4-phosphonobutyric acid selectively blocks two-way avoidance learning in the mouse

There seems to be ample evidence supporting the view that glutamate plays a significant role in the mammalian brain as a neurotransmitter. It is considered to be a likely transmitter candidate in one or more hippocampal pathways. Recently it has been visualized in excitatory, possibly glutamatergic, neurons in the hippocampus. Glutamate has been proposed to mediate memory formation. We wanted to see if blocking glutamate action by a specific glutamate antagonist could result in reduction of learning ability. 2-Amino-4-phosphonobutyric acid (APB) is an analogue of glutamic acid and has been used as a glutamate antagonist in electrophysiological studies on invertebrate neuromuscular junction, retina and hippocampus. We tested the influence of APB on the acquisition of two way avoidance learning in the shuttle box and on learning in the water maze. Our results show that intraperitoneal injection of APB led to a reduction in avoidance learning, whereas learning in the water maze was unaffected.

The distribution of neurons containing delta sleep-inducing peptide in the hippocampal formation

The distribution of neurons containing immunoreactive delta sleep-inducing peptide (IDSIP) in the hippocampal formation was investigated by immunocytochemistry. For this study, thirteen antisera to the peptide were used. Three antisera were found suitable for immunocytochemistry; the remainder gave only non-specific background staining. Neurons containing IDSIP were demonstrable in the subicular cortex immediately adjacent to CA1. No specific staining was found in other parts of the hippocampal formation including the pyramidal layer of Ammon's Horn and the dentate gyrus. These three antisera also stained cells in the cerebral cortex-primarily temporal, parietal and frontal regions--as well as in the ventral forebrain, although the number of cells in each area varied somewhat with the different antisera. The presence of IDSIP in neurons which constitute the major efferent pathway of the hippocampus, as well as in areas known to be functionally related to the hippocampus, i.e., the adjacent isocortices and ventral forebrain, suggests a possible role for the peptide in the regulation of behavior.

Regional synthesis of 14C-amino acids in cerebral cortex of the unanesthetized rat: simultaneous analysis by "push-pull" perfusion

Rats were surgically prepared with two sets of guide cannulae positioned bilaterally to rest below the superficial layer of the prefrontal, parietal and occipital regions of the cerebral cortex. Following post-operative recovery, two cortical sites were labeled simultaneously by microinjection of a 1.0 microliter volume of 5.0 microCi of D-[U-14C]-glucose. After a 20 min interval for incorporation into amino acids, two sites were concurrently perfused over an interval of 20 min by means of a push-pull cannula system with an artificial CSF solution at a rate of 25 microliters/min. A thin-layer chromatographic assay for separation of amino acid neurotransmitters was used to estimate the content in each sample of perfusate of labeled gamma-amino-butyric acid (GABA), glutamate, glutamine, aspartate and glycine. The results show that recovery of three amino acids, GABA, aspartate and glycine, was uniform among the three areas of the rat's cortex. However, the synthesis of labeled glutamate and glutamine was significantly lower within the parietal cortex in comparison to that of the prefrontal and occipital regions. Overall, the recovery of GABA was proportionally higher whereas glycine was lower than that of the other amino acids. The difference in the results from previous observations can be attributed to the distribution of amino acids in neurons and their terminals in deeper layers of cortex, which may give a different profile of release when epidural measurements are obtained.

Differential sensitivity of selected rat brain areas to excitatory neurotransmitters released by K+ depolarization

The depolarization of brain slices by high K+ concentrations in the presence of Ca2+ ions leads to the release from nerve terminals of endogenous excitatory neurotransmitters that interact with subsynaptic receptors controlling the neuronal membrane permeability to Na+ ions. This interaction has been studied by following the K+-evoked Ca2+-dependent 22Na+ efflux from 22Na+-preloaded slices from the cortex, striatum, hippocampus, cerebellum, substantia nigra and hypothalamus of the rat. The results indicate that the so-called N-methyl-D-aspartate receptor mediates the bulk of the excitatory transactions in the cortex and striatum whereas a subsynaptic glutamate receptor may mediate those in the hippocampus and cerebellum.

Role of presynaptic dopamine receptors in regulation of the glutamatergic neurotransmission in rat neostriatum

In experiments with the use of a push-pull cannula and simultaneous recording of electrical activity at the site of perfusion, the release of L-[3H]glutamic acid from rat neostriatum induced by K+-depolarization (60 mM K+ in perfusate) has been shown to be inhibited by replacing Ca2+ in the perfusion medium by Co2+. In contrast, release of L-[3H]glutamate induced by electrical stimulation of frontal cortex is enhanced by replacement of these cations. Application of dopamine (10(-5)-10(-3) M). apomorphine (10(-4) M) or beta-phenylethylamine (10(-3) M) as well as stimulation of the substantia nigra enhanced the basal release of L-[3H]glutamate. Haloperidol (10(-4) M) completely abolished the effects of apomorphine and beta-phenylethylamine, and partially abolished the effect of dopamine. The enhancement induced by apomorphine is strongly dependent on the presence of Na+ in the perfusion medium. On the other hand, apomorphine (10(-4) M) and beta-phenylethylamine (10(-3) M) inhibited the release of glutamate induced by electrical stimulation of the frontal cortex and that by K+-depolarization (the latter was shown for apomorphine). This inhibition is also abolished by haloperidol. A possible functional role of endogenous dopamine in the regulation of glutamatergic neurotransmission in rat neostriatum is discussed.

The reversal potential of excitatory amino acid action on granule cells of the rat dentate gyrus

The responses of granule cells to glutamate, aspartate, N-methyl-D-aspartate (NMDA), quisqualate and kainate applied by ionophoresis on to their dendrites in the middle molecular layer of the dentate gyrus were studied with intracellular electrodes using an in vitro hippocampal slice preparation. On passive depolarization 75% of the granule cells displayed anomalous rectification, which persisted in the presence of TTX and TEA but was eliminated by Co2+ or the intracellular injection of Cs+. Short ionophoretic applications of all the excitatory amino acids evoked dose-dependent depolarizations that were highly localized: movement of the ionophoretic electrode by as little as 10 microns could substantially change the size of the response. The depolarizations evoked by glutamate, asparatate, quisqualate and kainate were unaffected by TTX and Co2+. The depolarization evoked by NMDA was unaffected by TTX but markedly reduced by Co2+. Following intracellular injection of Cs+, neurones could be depolarized to +30 mV and the depolarizations produced by glutamate, quisqualate, NMDA and kainate reversed. The reversal potentials (E) were Eglutamate: -5.6 +/- 0.4 mV; ENMDA: 1.8 +/- 1.9 mV; Equisqualate: -3.9 +/- 1.9 mV; Ekainate: -4.6 +/- 2.0 mV. The excitatory post-synaptic potential (e.p.s.p.) evoked by stimulation of the medial perforant path could also be reversed and Ee.p.s.p. was -5.5 +/- 1.1 mV. The 6 mV difference between ENMDA and the equilibrium potential for the other exogenously applied excitatory amino acids and the statistically significant difference between ENMDA and Ee.p.s.p. (P less than 0.005; d.f.: 7) is consistent with our earlier hypothesis that both the transmitter released by the medial perforant path and exogenously applied glutamate are unlikely to interact with NMDA receptors.

A neural plasticity hypothesis of schizophrenia

An extensive research effort has failed, thus far, to conclusively identify a specific disease process (or processes) underlying the behavioral symptoms of schizophrenia. The present paper will entertain the hypothesis that the structural and functional plasticity of the brain can constitute a "nonspecific" biological etiology of schizophrenia. This plasticity need not be accompanied by infectious processes or gross alterations in neurotransmitter levels, enzyme activities, etc. that are specific to schizophrenia. The monkey isolation syndrome provides a precedent for a causal relationship between brain plasticity and pathological behavior. In a speculative manner, it will be demonstrated that neural plasticity concepts can be invoked to potentially explain several aspects of schizophrenia: the various types of behavioral symptoms exhibited by schizophrenics, the regional alterations in brain structure and function seen in chronic schizophrenics, the involvement of genetic and environmental etiological factors, the pharmacological support for the dopamine hypothesis, and the delayed onset of neuroleptic antipsychotic action. Considering the explanatory potential of neural plasticity concepts, a research program which focuses on these concepts seems warranted.

Structure - function relationships for gamma-substituted glutamate analogues on dentate granule cells

We previously demonstrated in the Schaffer collateral-CA1 region of the hippocampus that bath-applied agonists could be distinguished from antagonists among a group of acidic amino acid analogues by extracellular recording techniques. Here we report the use of the extracellular signs of agonist activity for discerning agonists and antagonists among several gamma-substituted glutamate analogues tested in the perforant path. The two-pathway composition of the perforant path offers the advantage over CA1 in that pathway-specificity, a postulated characteristic of antagonists, may be tested. By extracellular recording, D- and L-homocysteic acid, L-serine-O-sulfate, and L-2-amino-4-(5-tetrazolyl)-butanoic acid [L-glutamate tetrazole] were identified as agonists, and all 4 analogues were more potent than L-glutamate for inhibiting synaptic field potentials. Two previously identified antagonists, L-2-amino-4-phosphonobutyric acid and L-O-phosphoserine, exhibited the pathway-specificity and inhibitory kinetics consistent with properties expected for antagonists; both compounds detected 3 perforant path components with the same rank in sensitivity, suggesting that they are acting on the same set of receptors.

Blockade of amino acid-induced depolarizations and inhibition of excitatory post-synaptic potentials in rat dentate gyrus

Excitatory post-synaptic potentials (e.p.s.p.s) evoked by stimulation of the medial perforant path and depolarizations induced by excitatory amino acids were recorded from granule cells in the preparation of the hippocampal slice from the rat. The effects of (+/-)-2-amino-5-phosphonovalerate (APV), gamma-D-glutamylglycine (gamma DGG) and cis-2,3-piperidinedicarboxylate (PDA), antagonists of excitatory amino acids on these phenomena were compared. gamma DGG was the most effective antagonist of the e.p.s.p. Its action was reversible and not associated with any change in the passive membrane properties of the granule cells or in the apparent reversal potential of the e.p.s.p. Quantal analysis showed that the reduction in the e.p.s.p. paralleled the decrease in quantal size rather than quantal content, confirming a post-synaptic site of the action of gamma DGG. The potency of gamma DGG against the exogenous agonists was N-methyl-D-aspartate greater than kainate greater than or equal to quisqualate. APV had very little effect on the e.p.s.p. but was a selective antagonist of N-methyl-D-aspartate-induced depolarizations. PDA depolarized granule cells and increased their membrane input resistance. Although gamma DGG was a potent antagonist of both glutamate- and aspartate-induced depolarizations, no clear pattern of specificity could be found. The action of glutamate was unaffected by APV. These results indicate that the receptor for the transmitter at the synapses formed by the fibres of the perforant path with the granule cells is of the quisqualate and/or kainate type. The present data are consistent with the biochemical evidence that glutamate may be the endogenous transmitter at his synapse.

Ganglioside treatment improves recovery of alternation behavior after unilateral entorhinal cortex lesion

Exogenous gangliosides have been reported to enhance neurite formation in vitro and in vivo after damage to peripheral nerves. We report here the effects of ganglioside treatment on the course of recovery of alternation behavior that follows a unilateral lesion of the rat entorhinal cortex. The recovery of this function is known to parallel rapid synaptic reinnervation (collateral sprouting) into the partially denervated dentate gyrus of the hippocampus which previously received afferent input from the entorhinal region. Rats trained on an alternation behavior were subjected to a unilateral entorhinal lesion and subsequently given daily injections of total ganglioside (50 mg/kg, i.m.). Testing of the behavior continued for 2 weeks to assess the extent of behavioral impairment and the rate of recovery. Rats treated with gangliosides showed reduced behavioral impairment, accelerated recovery of the learned behavior, and final performance levels greater than controls. We hypothesize that the gangliosides may be interacting with regenerating neuronal membranes, either acting as receptors for trophic growth factors or altering membrane structure itself.

Quantitative autoradiography of L-[3H]glutamate binding to rat brain

A technique has been developed to investigate sodium-independent L-[3H]glutamate binding in rat brain sections using quantitative autoradiography and tritium-sensitive film. Binding is rapid (reaching equilibrium in 5 min) and reversible (having a t 1/2 of dissociation of 0.38 min). Glutamate apparently bound to a single population of sites with a Kd of about 1.0 microM. The pharmacology of this binding site is similar to that observed in homogenate studies. There is marked regional variation in the amount of glutamate bound. Of the areas analyzed in detail, the density of sites is greatest in stratum moleculare of hippocampus, followed by striatum and cortex.

Antagonists of synaptic and amino acid excitation of neurones in the cat spinal cord

In the spinal cord of the anaesthetized cat microelectrophoretically administered (+/-)-cis-2,3-piperidine dicarboxylate (2,3-PDA), (+/-)-cis-2,5-piperidine dicarboxylate (2,5-PDA), gamma-D-glutamylglycine (gamma DGG), beta-D-aspartyl-beta-alanine (beta DAA), (+/-)-2-amino-4-phosphonobutyrate (2-APB), (+/-)-2-amino-5-phosphonovalerate (2-APV) and (+/-)-2-amino-7-phosphonoheptanoate (2-APH) were assessed as antagonists of chemical excitation of dorsal horn interneurones and Renshaw cells by N-methyl-D-aspartate (NMDA), L-aspartate, quisqualate (QUIS), kainate and L-glutamate, and of monosynaptic and polysynaptic excitation by impulses in primary afferent fibres of muscle and cutaneous origin. Whereas polysynaptic excitation of interneurones was readily and reversibly depressed by 2-APV, 2-APH, beta DAA, gamma DGG and 2,3-PDA, all of which also reduced excitation by NMDA (and L-aspartate) more than that by QUIS (and L-glutamate), no selective antagonism of monosynaptic excitation could be demonstrated. In particular, 2,3-PDA, which depressed excitation by kainate to a greater extent than that by either QUIS or NMDA, appeared to have no effect on monosynaptic excitation. The results support the involvement of L-aspartate as the transmitter of some spinal excitatory interneurones, but none of the antagonists tested were considered suitable for assessing the role of L-glutamate as the transmitter of some spinal primary afferent fibres.

Receptors for the excitatory amino acids in the mammalian central nervous system

On the basis largely of neuropharmacological analysis, three different receptors mediating neuronal excitation can be identified. The first is activated by quisqualate and other "flexible" molecules including L-glutamate and appears to bind its ligands in a folded configuration. The second is excited by NMDA and has a more extended conformation, the spacing between the amino and the omega-carboxylate groups being the determinant of specificity. The third type accepts kainate and appears to possess a reactive site for the unsaturated side chain which is essential to the operation of this receptor. All three classes appear to be implicated in synaptic events [although some kainate receptors at least are certainly extra-synaptic (Watkins et al., 1981)] and each appears to activate different ionophores in neuronal membranes. Of the endogenous amino acids which may function as synaptic transmitters, L-glutamate and L-cysteate seem to react preferentially with quisqualate receptors (McLennan and Lodge, 1979), while L-aspartate is more of a mixed agonist capable of reaction both with quisqualate and with the NMDA types. Whether folate has a physiological role involving kainate receptors is unknown; and the same is true of any action possessed by quinolinate. The fact that there are amino acid excitants which are pharmacologically distinct from those reacting with any of the three best known receptors suggests that at least one more class of receptor may also exist, but no further information is available at the present time. Other sites with which the pharmacologically active acidic amino acids react are identifiable neurochemically in membrane preparations derived from tissues of the central nervous system. Kinetic studies and analysis of inhibition of sodium-independent binding indicate that there are sites which accept glutamate, others binding aspartate and a third which binds kainate. However, the first does not correspond completely to the quisqualate excitatory receptor, and NMDA does not react with any of the binding sites. It is difficult to conclude then that any of these binding sites can be fully identified with the excitatory receptors. Finally, there are a number of systems which in their patterns of activity again appear completely distinct, but which presumably mediate uptake of amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Response of Schaffer collateral-CA 1 pyramidal cell synapses of the hippocampus to analogues of acidic amino acids

Analogues of the putative excitatory transmitters aspartic acid and glutamic acid were tested for antagonism against stimulus-evoked activation of Schaffer collateral-CA 1 pyramidal cell synapses in slices of rat hippocampus. Responses to the analogues, applied via the superfusing medium, were extracellularly recorded. The compounds examined included D- and L-alpha-aminodicarboxylic acids, diaminodicarboxylic acids, phosphonate analogues of acidic amino acids, D- and L-gamma-glutamyl glycine, and the cis- and trans-isomers of piperidine 2,3-, and 2,4-dicarboxylic acid. Many of these compounds are known to be potent and selective antagonists for excitatory amino acids and a few excitatory pathways. In this hippocampal pathway most of these analogues showed relatively low and similar potency. The most potent antagonist uncontaminated with agonist activity was D-alpha-aminosuberate, with an apparent antagonist dissociation constant (Kd) of 3 mM. Only 5 of the analogues, 3 of the piperidine dicarboxylates, kainic acid, and L-alpha-aminopimelic acid, reduced the amplitude of the extracellularly recorded field potentials more than 30% at 0.5 mM. However, all of the others reduced the potential by more than 30% at 5 mM. Most analogues also evoked extracellular responses which can be attributed to depolarization of the pyramidal neurons. Agonist activity was particularly strong among the most potent analogues. These results contrast with the responses documented by others for the N-methyl-D-aspartate receptor of the dorsal-ventral root excitatory pathway of the spinal cord in which the higher homologues tested here were the most potent antagonists, and the D-isomers were more potent than the L-isomers. It also contrasts with the response of the perforant path synapses to granule cells of the dentate gyrus in which the portion derived from the lateral entorhinal cortex is sensitive to L-2-amino-4-phosphonobutyric acid. Thus the Schaffer-CA 1 pyramidal cell synaptic field utilizes a novel excitatory transmitter receptor which interacts detectably but only weakly with a variety of acidic amino acids with potent specific inhibitory action for receptors elsewhere in the central nervous system.

Chloride ions enhance L-glutamate binding to rat brain synaptic membranes

Chloride ions increased L-glutamate (L-Glu) binding to synaptic membranes. The binding was saturable and resulted in a 2.5-fold enhancement at concentrations of 20--40 mM chloride. Sodium and potassium ions inhibited only chloride stimulated L-Glu binding. Calcium ions also increased L-Glu binding but this was observed only in the presence of chloride. The anion selectivity of the enhancement of L-Glu binding was similar to that reported for the membrane chloride channel, suggesting that some L-Glu binding sites may be associated with this channel.

Radioautographic analysis of [14C]2-deoxyglucose uptake in hippocampal formation of the rat during enforced locomotor activity-induced theta

The [14C]2-deoxyglucose ( [14C]2-DG) uptake in the hippocampal formation of the rat was studied following enforced locomotor activity-induced theta (M theta). M theta was found to be essentially associated with an increase in 2-DG uptake in the stratum oriens of the hippocampal CA1-CA2 areas. These data contrast with our previous findings that physostigmine-induced theta (I theta) is specifically associated with a decrease in 2-DG uptake in the stratum lacunosum moleculare of hippocampal CA1-CA2 areas. When both of our sets of radioautographic data are considered together, M theta and I theta appear to have a distinct neurophysiological basis.

Characterization of neurotransmitter receptors in the rat hippocampal formation

The hippocampal formation has been extensively research in terms of its putative neurotransmitters, anatomical connections, and behavioral relevance. An aspect of importance is the assessment of apparent neurotransmitter receptors by using receptor binding assays. In the present study, such assays were done in vitro to investigate alpha 1-adrenergic, alpha 2-adrenergic, beta-adrenergic, muscarinic cholinergic, benzodiazepine, and opiate receptors in the rat hippocampal formation. The corresponding radioligands for these receptors were [3H]prazosin, [3H]p-aminoclonidine, [3H]dihydroalprenolol, [3H]quinuclidinyl benzilate, [3H]flunitrazepam, and [3H]naloxone. An analysis of the binding parameters for the ligands indicated saturable binding of a high affinity and the following rank order of maximal binding capacities: [3H]flunitrazepam greater than [3H]quinuclidinyl benzilate greater than [3H]naloxone greater than [3H]p-aminoclonidine greater than [3H]prazosin greater than [3H]dihydroalprenolol. Competition experiments with pharmacologic agonists and antagonists confirmed the specificity of each ligand. The results are integrated with information on other types of receptors and with neurotransmitter concentrations, and discussed in terms of hippocampal function.

Systemic injection of kainic acid: effect on neurotransmitter markers in piriform cortex, amygdaloid complex and hippocampus and protection by cortical lesioning and anticonvulsants

Systemic injection of kainic acid (12 mg/kg) in rats induces a well established pattern of neuronal lesions in different brain regions. These lesions are accompanied by changes in neurotransmitter markers. In the piriform cortex and amygdaloid complex, the kainic acid lesion was accompanied by a reduction in the high affinity uptake of glutamate and in the activities of glutamate decarboxylase and choline acetyltransferase, whereas in the hippocampus there was a reduction in the high affinity uptake of glutamate and in glutamate decarboxylase activity. Hemidecortication, hemitransection, a caudal knife cut in the cortex, or treatment with diazepam, all protected against the effects of kainic acid in the piriform cortex and amygdaloid complex but not in the hippocampus. Diphenylhydantoin had no effect on the neurotoxicity of kainic acid. The results indicate that the neurotoxic effects of kainic acid in the piriform cortex and amygdala are dependent on an intact cortical structure, probably due to a dependence on specific excitatory circuitry. The neurons involved may be glutamergic/aspartergic.

Actions of D and L forms of 2-amino-5-phosphonovalerate and 2-amino-4-phosphonobutyrate in the cat spinal cord

The separate optical enantiomers of 2-amino-5-phosphonovalerate (APV) and 2-amino-4-phosphonobutyrate (APB) have been tested for their ability to modify amino acid-induced and synaptic excitation of cat spinal neurones. D-(-)-APV was a highly potent and selective antagonist of amino acid-induced and synaptic excitation. Polysynaptic excitation was more susceptible to antagonism by D-APV than was monosynaptic excitation. It was considered likely that the depression of synaptic excitation by D-APV was due to the blockade of an excitatory amino acid transmitter acting at N-methyl-D-aspartate (NMDA) receptors. L-(+)-APV showed a relatively weak amino acid and synaptic blocking activity, which was similar in character to that of D-APV, and which may have been due to a slight residuum of the D isomer in the sample of the L form used. D-(-)-APB was a weak and relatively non-selective antagonist of amino acid-induced responses. In contrast, L-(+)-APB either had no effect or, at higher concentrations, enhanced these responses. Both isomers depressed synaptic responses in a proportion of the cells tested, the L form being the more potent isomer in producing this effect. Monosynaptic and polysynaptic excitations were both susceptible to this type of action. The depression of synaptic excitation by D-APB may have been due in some cases to the blockade of an excitatory amino acid transmitter. However, it is unlikely that the synaptic depressant action of L-APB is due to this mechanism.

Brain neurotransmitter turnover correlated with morphine-seeking behavior of rats

Neurochemical substrates of intravenous opiate self-administration were investigated in rats using littermate controls for vehicle and passive morphine infusion. The rates of turnover of the putative neurotransmitters, dopamine, norepinephrine, serotonin, gamma-aminobutyric acid, aspartate and glutamate were concurrently measured in eleven brain regions of rats intravenously self-administering morphine and yoked-morphine or yoked-vehicle infused littermates. The passive infusion of morphine resulted in significant changes in the rates of turnover of the biogenic monoamine and amino acid neurotransmitters in six brain regions with the caudate nucleus-putamen-globus pallidus showing the most changes. The contingent infusion of morphine resulted in changes in utilization rates that were generally greater in both magnitude and number than the effects of the drug itself. Twenty-nine significant changes were observed in the self-administering group with most changes occurring in limbic structures. The neurotransmitter turnover rate changes resulting from contingent administration suggest that the drug administration environment is an important factor that should be considered in studies of interactions between drugs and neuronal systems.

Amino acid neurotransmitter utilization in discrete rat brain regions is correlated with conditioned emotional response

The content and utilization of amino acid neurotransmitters were evaluated in discrete brain areas of rats exposed to a conditioned emotional response (CER) procedure and in control groups which received either equivalent yoked shock history (shock only) or compound stimulus presentation (tone only). On test day, CER animals suppressed responding and exhibited anxious behavior after presentation of the CS, while shock only and tone only control groups, or CER animals which received an acute dose of diazepam prior to testing, did not suppress. Few changes were observed in the content of amino acids, suggesting that the behavioral manipulations were acting within normal physiological limits. On the other hand, numerous changes were observed in the utilization (turnover, metabolism) of the amino acid neurotransmitters. The effects of a history of shock presentation (shock only versus tone only) were persistent long after the conditioning sessions were terminated, and resulted in decreased turnover of the amino acids in many areas. CER conditioning-emotion (CER versus shock only) produced an increase in the turnover of aspartate and glutamate in many structures, while changes in GABA turnover were generally limited to decreases in limbic areas. If CER represents an animal model of anxiety, these observations may suggest roles for neurons which utilize amino acids in mediating or responding to emotional components of the paradigm.

The effects of a series of omega-phosphonic alpha-carboxylic amino acids on electrically evoked and excitant amino acid-induced responses in isolated spinal cord preparations

1 The depressant actions on evoked electrical activity and the excitant amino acid antagonist properties of a range of omega-phosphonic alpha-carboxylic amino acids have been investigated in the isolated spinal cord preparations of the frog or immature rat. 2 When tested on dorsal root-evoked ventral root potentials, members of the homologous series from 2- amino-5-phosphonovaleric acid to 2-amino-8-phosphonooctanoic acid showed depressant actions which correlated with the ability of the substances to antagonize selectivity motoneuronal depolarizations induced by N-methyl-D-aspartate. 3 2-Amino-5-phosphonovalerate was the most potent substance of the series giving an apparent KD of 1.4 microM for the antagonism of responses to N-methyl-D-aspartate. 4 A comparison of the (+)- and (-)-forms of 2-amino-5-phosphonovalerate indicated that the N-methyl-D-aspartate antagonist activity and the neuronal depressant action of this substance were both due mainly to the (-)-isomer. 5 The (-)- and (+)-forms of 2-amino-4-phosphonobutyrate had different actions. The (-)-forms of this substance had a relatively weak and non-selective antagonist action on depolarizations induced by N-methyl-D-aspartate, quisqualate and kainate and a similarly weak depressant effect when tested on evoked electrical activity. The (+)-form was more potent than he (-)-form in depressing electrically evoked activity but did not antagonize responses to amino acid excitants. At concentrations higher than those required to depress electrically evoked activity, the (+)-form produced depolarization. This action was blocked by 2-amino-5-phosphonovalerate.

GABAergic modulation of learned helplessness

Previous studies have demonstrated the GABAergic system in the hippocampus to be a major controlling factor in the reversal of learned helplessness by antidepressants. In the present work, animals in which learned helplessness (LeH) was induced by exposure to uncontrollable electric shock demonstrated a decreased depolarization-induced release of GABA relative to controls, and an increased release of hippocampal glutamate. Injection of bicuculline but not glutamate into the hippocampus produced a behavioral state identical to that induced by uncontrollable shock, but had no influence on pain sensitivity. When flux through the "small" hippocampal pool of GABA was determined, chronic treatment with imipramine or iprindole, but lorazepam or chlorpromazine elevated this measure. These three findings along with those of prior experiments, suggest a controlling role for GABA in the learned helplessness model of depression.

Glutamate binding in the rat cerebral cortex during ontogeny

Two binding sites for L-glutamate have been identified on adult rat brain cortical membranes. One of these sites is Na+-dependent with Kd of 1.3 micro M and a Bmax of 210 pmol/mg protein. The other is Na+-independent with a Kd of 0.37 micro M and Bmax of 6.2 pmol/mg protein. There is a sharp rise in total number of Na+-independent sites per cortex up to 20 days postnatally followed by a more gradual rise to adult levels at 50 days. Na+-dependent binding is also low at birth rising to a peak at 20 days followed by a drop in total levels of binding to 30 days and then a very sharp rise up to 50 days. The kinetics of binding at 20 days gives a Kd for the Na+-dependent site of 1.77 micro M and a Bmax of 82 pmol/mg protein. The Na+-independent site at 20 days has a Kd = 1.3 micro M and Bmax of 8.47 pmol/mg protein. The ability of several acidic amino acid analogues to displace specifically bound L-glutamate was investigated by estimating IC50 values at 20 and 50 days of age. The Na+-independent site is stereospecific for L-glutamate at both ages, but will also interact with L-aspartate at 20 days. The Na+-dependent site has a similar affinity for L- and D-glutamate and L-aspartate at 50 days. The L-glutamate analogue kainate will not displace any bound L-glutamate.

Kainic acid produces depolarization of CA3 pyramidal cells in the vitro hippocampal slice

Kainic acid (KA) (10(-6)-10(-8) M) reversibly depolarized CA3 pyramidal cells when applied topically to the apical dendritic area of these cells in the hippocampal slice. The magnitude of membrane depolarization and the time to recovery of resting membrane potential were concentration-related. Application of 10(-5) M KA produced complete membrane depolarization which did not recover in baseline levels. Unlike CA3 neurons cells from the CA1 region were unaffected by KA (10(-6)-10(-8) M). However, 10(-5) M KA also proved effective in depolarizing CA1 cells.

Micromolar L-2-amino-4-phosphonobutyric acid selectively inhibits perforant path synapses from lateral entorhinal cortex

Transverse slices of the rat hippocampus were used to examine the ability of phosphonate analogues of acidic amino acids to inhibit perforant path synaptic transmission. Micromolar concentrations of L-2-amino-4-phosphonobutyric acid (APB), an analogue of L-glutamic acid, inhibited transmission from the lateral entorhinal cortex. Two other less-sensitive components were detected in projections from the medial entorhinal cortex. The component from the lateral entorhinal cortex showed high stereospecificity for the L-isomer of APB and was relatively insensitive to phosphonate homologues of shorter and longer chain length.

Is glutamate a trigger factor in epileptic hyperactivity?

The effects of glutamate and aspartate antagonists were studied on limb-jerk frequency and EEG patterns in rats made epileptic by cobalt implantation in the sensorimotor cerebral cortex. The results of this study were as follows: (1) alpha-Amino-4-phosphonobutyric acid (0.055--1.10 mM) decreased or completely prevented the epileptic manifestations. The effect was reversible 30 min after washing with artificial CSF solution. (2) DL-Pyroglutamic acid completely abolished the myoclonic jerks and EEG spikes. (3) alpha-Amino-5-phosphonovaleric acid and alpha-D-amino-adipic acid significantly reduced the frequency of epileptic spikes and myoclonic jerks. (4) Other analogs, alpha-amino-3-phosphonopropionic acid (0.6 mM), alpha-amino phosphonocaproic acid (0.95), and hydroxy-3-amino pyrrolidone-2 were without effect. (5) Glutamate itself did not decrease the epileptic manifestations.

Effects of spinal transection on presynaptic markers for glutamatergic neurons in the rat

To evaluate the hypothesis that glutamic acid may be the neurotransmitter of descending, excitatory supraspinal pathways, the uptake and release of L-[3H] glutamate and the levels of endogenous glutamate were measured in preparations from rat lumbar spinal cord following complete mid-thoracic transection. Following transection, the activity of the synaptosomal high-affinity glutamate uptake process was increased in both dorsal and ventral halves of lumbar cord between 1 and 14 days after transection and returned to control levels by 21 days posttransection. At 7 days, the increased activity of the uptake process for L-[3H]glutamate resulted in elevation of Vmax with no significant alteration in KT as compared to age-matched controls. Depolarization-induced release of L-[3H]glutamate from prelabeled slices did not differ significantly from control in the lesioned rat except at 21 days after lesion when the amount of tritium release was significantly greater in the transected preparations than in control. Amino acid analysis of the lumbar cord from control and transected rats indicated only a 10% decrease in the level of endogenous glutamate and no alterations in the concentration of GABA and glycine 7 days after lesion. These findings do not support the hypothesis that glutamate serves as a major excitatory neurotransmitter in supraspinal pathways innervating the lumbar cord of the rat.

Lateral olfactory tract transmitter: glutamate, aspartate, or neither?

Aspartate and glutamate are the principal candidates for the excitatory neurotransmitter released by the lateral olfactory tract (LOT) in prepyriform cortex of the rat. Identity of action of the natural transmitter with exogenous glutamate and/or aspartate, however, has not yet been demonstrated. We show that bath-applied 2-amino-4-phosphonobutyric acid, a presumed specific glutamate antagonist, blocks LOT-stimulated prepyriform field potentials and single unit activity but not the single unit response to ionophoretically applied glutamate or aspartate in rat olfactory cortex slices. These results suggest that neither aspartate nor glutamate is the LOT transmitter. Responses to ionophoretically applied N-methyl-DL-aspartate, kainic acid, and DL-homocysteate were clearly decreased by 2-amino-4-phosphonobutyric acid. This suggests that these agents, usually presumed to be aspartate or glutamate agonists, act at different receptors than aspartate and glutamate.

Characterization of two [3H]glutamate binding sites in rat hippocampal membranes

The specific binding of L-[3H]glutamate was investigated in the presence and the absence of sodium ions in freshly prepared membranes from rat hippocampus. Sodium ions were found to have a biphasic effect; low concentrations induced a marked inhibition of the binding (in the range 0.5-5.0 mM), whereas higher concentrations resulted in a dose-dependent stimulation of binding (in the range 10-150 mM). These results permit the discrimination of two binding sites in hippocampal membranes. Both Na+-independent and Na+-dependent binding sites were saturable, exhibiting dissociation constants at 30 degrees C of 750 nM and 2.4 microM, respectively, with Hill coefficients not significantly different from unity, and maximal number of sites of 6.5 and 75 pmol/mg protein, respectively. [3H]Glutamate binding to both sites reached equilibrium between 5 and 10 min and was reversible. The relative potencies of a wide range of compounds, with known pharmacological activities, to inhibit [3H]glutamate binding were very different for the Na+-independent and Na+-dependent binding and suggested that the former sites were related to post-synaptic glutamate receptors, whereas the latter were related to high-affinity uptake sites. This conclusion was also supported by the considerable variation in the regional distribution of the Na+-dependent binding site, which paralleled that of the high-affinity glutamate uptake; the Na+-independent binding exhibited less regional variation.

2-Amino-5-phosphonovalerate (2APV), a potent and selective antagonist of amino acid-induced and synaptic excitation

A new compound, 2-amino-5-phosphonovaleric acid (2APV) is the most potent and selective N-methyl-D-aspartate (NMDA) receptor antagonist yet tested. As with other compounds of this type, it blocks L-aspartate and dorsal root-evoked excitation of spinal neurons, but is without effect on the cholinergic excitation of Renshaw cells evoked by exogenous acetylcholine or ventral root stimulation. The high potency and selectivity of this compound should prove to be of great value in investigations of the amino acid receptor types involved in synaptic excitation.

The role of epileptic activity in hippocampal and "remote" cerebral lesions induced by kainic acid

Kainic acid (KA) was injected systemically, intracerebroventricularly (i.c.v.) and focally in the amygdala and other deep brain structures in the rat. EEG and behavioral changes were studied in relation to the neuropathology which developed subsequently. Following intra-amygdaloid KA injection, diazepam blocked the epileptic events induced by the toxin, and abolished the neuronal loss usually seen in the lateral septum, claustrum, and contralateral cortex and hippocampus. The lesions in medial thalamic structures and ipsilateral hippocampus were also reduced by diazepam. Prior transection of the perforant path ipsilateral to the KA injection also decreased the severity of the electrographic and motor effects of the toxin and similarly reduced the extent of distant ("remote") pathological brain damage. Neither diazepam nor perforant path transection reduced the damage at the site of KA injection. Kainic acid (0.4-2 microgram) injected into the bed nucleus of the stria terminalis (BST) or the medial septum produced seizures with a longer latency and little brain damage outside the injection site. In contrast, intrastriatal KA injections were followed by ipsilateral hippocampal lesions. i.c.v. Injection of KA (0.4-1.6 microgram) produced a complex syndrome which included bilateral exophthalmos, mydriasis, foaming, tremor of the vibrissae, and paw and body tremor. The pattern of brain damage resembled that seen following intra-amygdaloid administration of the toxin. In addition, however, there was a bilateral necrosis of the pyriform and prepyriform cortices up to the rhinal fissure. Systemic administration of diazepam (i.p.) reduced the extent of the damage and in particular completely prevented the cortical damage. Systemic administration of KA (9-15 mg/kg i.p.) readily produced motor and EEG seizures similar to those seen after intra-amygdaloid injection of the toxin. The pattern of brain damage was however more symmetrical than that which followed focal i.c.v. injection of the toxin and included necrosis of the pyriform cortex. It is concluded that spread of seizure activity from the injection site plays a crucial role in the induction of "remote" brain damage after focal intracerebral injections.

Blockade of high frequency rhythmical slow activity by intrahippocampal injection of a glutamic acid antagonist

Intrahippocampal injections of glutamic acid diethylester (GDEE) were performed in rats in order to study a possible implication of glutamate in the mediation of high frequency (8-12 Hz) rhythmical slow activity (RSA) elicited by 100 Hz electrical stimulation of the dorsomedial hypothalamus. This 8-12 Hz RSA was selectively and reversibly suppressed in hippocampal EEG 6 min and 15 min after 150 micrograms or 400 micrograms of GDEE, whereas no modifications were found in the low frequency RSA (6-8 Hz) evoked in hippocampus during exploratory behavior.

Regulation of hippocampal glutamate receptors: evidence for the involvement of a calcium-activated protease

Specific [3H]glutamate binding to rat hippocampal membranes and the calcium-induced increase in this binding are markedly temperature-sensitive and are inhibited by alkylating or reducing agents as well as by various protease inhibitors. N-Ethylmaleimide, chloromethyl ketone derivatives of lysine and phenylalanine, and tosylarginine methyl ester decrease the maximum number of [3H]glutamate binding sites without changing their affinity for glutamate. Preincubation of the membranes with glutamate does not protect the glutamate "receptors" from the suppressive effects of these agents. The proteases trypsin and alpha-chymotrypsin increase the maximum number of [3H]glutamate binding sites. The effects of calcium on glutamate binding are different across brain regions. Cerebellar membranes are almost insensitive whereas hippocampal and striatal membranes exhibit a strong increase in the number of binding sites after exposure to even low concentrations of calcium. These results suggest that an endogenous membrane-associated thiol protease regulates the number of [3H]glutamate-associated thiol protease regulates the number of [3H]glutamate binding sites in hippocampal membranes and that this is the mechanism by which calcium stimulates glutamate binding. The possibility is discussed that the postulated mechanisms participate in synaptic physiology and in particular may be related to the long-term potentiation of transmission found in hippocampus under certain conditions.

Characterization of specific, high-affinity binding sites for L-[3H]glutamic acid in rat brain membranes

L-[3H]Glutamic acid binds reversibly to rat brain membranes with high affinity. Specific binding is linear with tissue concentration and has a pH optimum at neutrality. Saturation isotherms reveal anomolous kinetics of specific binding with an high affinity site with a KD of 11 nM and a lower affinity site with a KD of 80 nM; the Scatchard plots intercept at a common bound/free ratio. Hill plots of the complete saturation isotherms have a slope of 1.0. There are marked regional differences in the distribution of binding sites in rat brain: parietal cortex, frontal cortex, hippocampus greater than striatum greater than thalamus greater than cerebellum, pons-medulla and hypothalamus. Except for a small amount of specific binding in heart, other peripheral tissues do not exhibit specific binding of L-[3H]glutamic acid. Several amino acids with neuroexcitatory effects inhibit the specific binding: L-glutamic acid greater than L-aspartic acid and D,L-homocysteic acid greater than D-glutamic acid and L-cysteine sulfinic acid; related amino acids without neuroexcitatory effects do not inhibit specific binding. Reputed antagonists of glutamate-induced neuronal depolarization block specific binding: alpha-aminoadipic acid greater than 2-amino,4-phosphonobutyric acid greater than glutamate diethylester. Prior kainate lesion of the neurons intrinsic to the striatum results in a 45% decrement in specific binding of L-[3H]glutamic acid whereas cortical ablation, which causes degeneration of a cortical-striatal glutamatergic projection and reduces striatal glutamate synaptosomal uptake, does not affect specific binding. These results are compatible with the interpretation that the binding of [3H]glutamic acid occurs at excitatory receptors on neurons.